Protecting the content of programs sent by satellite, cable or radio is in fact essential for the operators. The latter being remunerated by subscriptions, a system which can easily be pirated results in a loss of earnings. The transmission of scrambled or encrypted data is well known in the field of pay television systems, where the scrambled digital audiovisual data are typically sent by conventional transmission means to a certain number of subscribers. The subscribers have a receiver/decoder capable of unscrambling the data in order to make them capable of display. Receivers/decoders include in particular set-top boxes, where the decoder functions in combination with a receiver physically separate from the decoder, the said receiver being able to include additional functions, such as a system for browsing on a network, the Internet for example, a video recorder, a television etc.
In pay television systems, the scrambled data are transmitted together with a control word for unscrambling the digital data. The control word itself is scrambled with an operating key and transmitted in scrambled form. The scrambled data and the scrambled control word are received by the receiver/decoder having an equivalent to the operating key necessary for unscrambling the control word and then unscrambling the data transmitted. Conditional access therefore in general uses information dedicated to conditional access within the data stream and a conditional access system capable of using this information.
Current boxes are protected by a smart card. The box sends to the smart card coded information, the information for conditional access, in the data stream received. The smart card then decides to send or not a key for unscrambling the program. Such a box is known from the document EP 1 182 874.
The invention relates to the following considerations:
In general, in the state of the art, all the protection of the box lies in the smart card. Firstly, communication between the smart card and the box is the weak point in the circuit. This communication is not secure or is protected only by software, which means that the protection is weak. Consequently it is possible to listen to this communication in order to determine the key allowing unscrambling. This key may then be used directly in order to pirate other boxes. Secondly, the fact that all (or almost all) protection is based on a separable and isolatable element of the box facilitates the work of pirates, who because of this do not have to seek the protection in often complex boxes.